Virtually all ventilation ducts require one or more elbow bends to maneuver around obstacles or join ducts extending at different angles. Curved ducts are especially important on ships where duct diameter, angle and shape often change every few feet. The ventilation ducts are typically installed last and must be maneuvered around and through existing structures. Further, since regulations do not allow soft-sided, flexible ducts, each change in the duct diameter, angle or shape must be accomplished with rigid material which is carefully measured and cut to establish the proper change.
One of the most common methods of elbow formation involves the layout of the elbow as individual segments each having a longitudinal axis and a lateral axis normal to each other. The layout, hereinafter referred to as a development, establishes the dimensions of each segment which is individually cut from a blank. The separate segments, known as clam shells, are joined together by crimping or welding. This method is labor-intensive and slow.
A common method for forming straight duct of circular cross section is to wind it helically from a continuous blank having straight boundaries, that is, edges. No elbows are formed when the blank has straight boundaries.
Heiman, in U.S. Pat. No. 4,287,742, describes an apparatus which manufactures a curved conduit by helically winding it from a continuous blank whose edges have been cut in a particular pattern. However, as disclosed, e.g., in Column 8, the opposing boundaries of the blank simply differ in phase by 180 degrees: when such a blank is helically wound a conduit with double curvature results. That is, the apparatus as disclosed is capable only of manufacturing a twisted conduit having an arcuate axis that does not lie in a single plane. Such a twisted conduit is rarely used in the industry, and a method of altering the shape of the blank to form an elbow having a single curvature is not disclosed.